This invention relates to a bonding structure for timepieces, and in particular to a urethane (meta) acrylate series photo-stiffening bonding agent for bonding parts of a timepiece to provide a waterproof structure.
There are many conventional structures suitable for providing a waterproof timepiece. FIGS. 1 and 2 show such conventional structures in sectional view. It is desirable to miniaturize these watches and provide thin, waterproof cases. Such conventional cases generally include a synthetic resin gasket 31 as a sealing agent between a watch glass 32 and a case band 33 by inteference fit. In FIG. 2 glass bezel 38 is disposed intermediate case band 33 and glass 32 and synthetic resin gasket 31 is engaged between glass 32 and glass bezel 38. To provide a watertight seal, substantial pressure is exerted on gasket 31 by glass plate 32 and a bearing wall 45 of case band 33 or a bearing wall 41 of glass bezel 38.
Synthetic resin gasket 31 is generally formed from a fluoroethylene resin or the like. Case band 33 or glass bezel 38 must have bearing wall 45 or 41 respectively, of sufficient height to exert pressure on gasket 31. Case band 33 or glass bezel 38 must be large enough and properly shaped or they will deform from the stress generated by pressing on synthetic gasket 31. Gasket 31 must have a height B of between about 0.6 to 1.0 mm and a width A of gasket 31 must be between about 0.2 and 0.4 mm in order to provide the desired binding force and sealing properties. Since case band 33 or glass bezel 38 which hold glass 32 must be sufficiently strong to retain the needed stress of gasket 31, the height and width of case band 33 and glass bezel 38 tend to be large. This limits the extent to which a watch can be miniaturized or be made thin.
Watch cases formed using these synthetic resin gaskets have other drawbacks in addition to the limits on shape. An individual pre-formed gasket must be used for each particular timepiece case design. Thus, a great number of different sizes and shapes must be available. This interfers with both the convenience of manufacturing the cases and the ease of repair. Use of a synthetic resin gasket always requires a glass bearing wall which interfers with presenting several types of design. For example, it is difficult to provide a rectangular shape.
When a watch case is constructed with a waterproof gasket, several other components must be mounted to the case. As shown in FIGS. 1 and 2, a pipe 34 to receive a watch stem extends through watch case 33. Pipe 34 is generally fixed to case band 33 by a conventional UV bonding agent, or solder. A timepiece case back 35 and a core 36 are typically fixed by soldering. The seam between case band 33 and case back 35 is generally sealed with a water resistant packing 37. The seam between glass bezel 38 and case band 33 in FIG. 2 are generally made water-tight by including water resistant packing 39 therebetween.
Timepiece cases constructed in the above manner are deficient due to shortcomings in the conventional UV bonding agents used to bond the components of a timepiece case together. Special apparatuses are required to work with these adhesives and the adhesive force decreases with exposure to hot water as shown in FIG. 9. Further, conventional UV setting bonding agents do not provide waterproof seals.
Conventional epoxy-type bonding agents have been tried, but they also have drawbacks. They require that a chief agent and a stiffening agent be mixed. Production is delayed because substantial time must elapse before the epoxy bonding agents set. Although epoxy bonding agents can prevent moisture from damaging the timepiece, they do not provide a satisfactory waterproof seal under water pressure of 2 to 3 atmospheres. Finally, conventional epoxy-type adhesives are insufficiently resistant to heat. They are not durable enough and interfere with mass production techniques which increases the cost of manufacturing.
Accordingly, it is desirable to provide a bonding agent for a timepiece case which will overcome the fabrication, design and watrproof problems associated with the prior art case constructions.